18 AN ECONOMIC STUDY OF FIELD MICE. 
provinces and as the cultivated areas in the West arc extended under 
the stimulus of the United States Reclamation Service, the danger 
of serious ravages by meadow mice will increase rather than diminish. 
II. THE PRAIRIE MOUSE. 
Microtu8 ochrogaster Wagner. 
The prairie mouse is probably second only to the common meadow 
mouse in the extent of its injury to crops. Outwardly it differs hut 
slightly from the latter species. Its tail is shorter (less than twice 
the length of the hind foot) and its fur is coarser. Tn winter the 
pelage is grayer. The color of the underparts shades into a huh 1 ' 
or cinnamon. The contrast between the upper and lower parts of 
the tail is much sharper than in the common meadow mouse. The 
foot pads are 5, and the number of mamma? G (2 pectoral and '4 ingui- 
nal). Its average measurements are about as follows: Length, 155 
mm. (6 inches) : tail vertebra 1 . 38 mm. (1.5 inches) ; hind foot, 21 
mm. (0.82 inch). 
The prairie mouse occurs in southern Wisconsin, in Indiana. Illi- 
nois, Iowa. Missouri. Nebraska, Kansas, and a part of Oklahoma. 
It lives in the open prairie country, mainly in the Upper Austral zone. 
Thus it is much more likely to invade crops than if its natural 
habitat were in swamps. I have found it on the borders of corn and 
cane fields and in native meadoAvs, as well as in cultivated clover and 
alfalfa fields. It seems especially partial to fields that have been 
allowed to lie fallow for several seasons. The soft mixed annual 
grasses and weeds that partly replace the original prairie cover 
seem to furnish it congenial surroundings. Close grazing of the open 
ranges tends to drive out voles, but when ranges are not closely pas- 
tured, so that an abundance of old grass is left, prairie mice soon 
become numerous and appreciably reduce the amount of forage. 
In the same manner the prairie mouse invades pastures and neg- 
lected orchards whenever dry grass is permitted to. accumulate and 
remain over winter. If no crops are near, the animals subsist on 
wild herbage, roots, and seeds; but when cultivated crops are acces- 
sible their trails soon extend far into the tilled fields. 
Xests of prairie mice usually are less bulky than those of the com- 
mon meadow mouse, but are built in a similar way and in like situa- 
tions. The number of young at a birth is usually three or four, 
rarely five or six. In ordinary seasons the first litter is born in April, 
but in dry, warm springs the time may be fully a month earlier. The 
number of litters in a season varies with climatic and other condi- 
tions. On the whole, prairie mice multiply less rapidly than meadow 
mice, since the number of young at a birth is smaller, and the long 
summer droughts and extreme winters of the interior prairies of the 
West often limit reproduction. 
